story #2967 - Serialize gitolite events

Sevral concurrent processes accessing the same gitolite admin repository
in the same time seems to lead to race conditions and sometime might
crash gitolite authorized_keys.

Furthermore, as the number of git repositories grow, modifications made on
gitolite admin repo takes time to push (compile time in gitolite to generate
the permission schema).

With this change, all gitolite admin operations (ssh keys modifications,
creation of repository, deletion, update of description, update of permissions,
update of mailing list details, move to gerrit, etc) are done one after
another (serialized), in background (system event).

For end user, it means a UI with better reactivity but a little bit of delay
to actually get the things propagated at git/gitolite level (usually a couple
of minutes).

Change-Id: I5d58652690feaf2c756b04db30889639334aca40
Merge commit 'refs/changes/59/359/6' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  story #2967 - Serialize gitolite events

Conflicts:
	plugins/git/include/Git/RemoteServer/GerritServerFactory.class.php
	plugins/git/include/autoload.php
	plugins/git/include/gitPlugin.class.php
	plugins/git/site-content/en_US/git.tab
Tuleap 5.12.99.23